Implantable cardiac stimulators in the form of cardiac pacemakers or cardioverter/defibrillators are known in the prior art. Such cardiac stimulators are usually connected to electrode lines, which have stimulation electrodes and optionally additional defibrillation electrodes in the immediate vicinity. A cardiac pacemaker can deliver an electric stimulation pulse to the muscle tissue of a cardiac chamber via a stimulation electrode to thereby induce a stimulated contraction of the cardiac chamber, provided the stimulation pulse is of sufficient intensity and the myocardial tissue (myocardium) is not at that moment in a refractory phase. Electrode lines having relatively small-area stimulation electrodes are generally used for this purpose, since it is usually sufficient if only a small portion of the myocardium of the cardiac chamber is initially stimulated when triggering stimulated contraction. Within the context of this discussion, such stimulated contraction of the cardiac chamber will be referred to as a “stimulated” event, whereas if there is a natural contraction of the cardiac chamber, this is regarded as an “intrinsic” or “natural” cardiac activity. For example, contraction of the right atrium of the heart can be referred to as an atrial event, which may be a natural atrial event, or in the case of an atrial pacemaker, it may also be a stimulated atrial event. Natural (intrinsic) and stimulated left-ventricular and right-ventricular events can be distinguished in the same manner.
Local stimulation of the myocardium propagates from the stimulus site by stimulus conduction in the myocardium, and leads to depolarization of the muscle cells and thus to contraction of the myocardium. After a short period of time, there is repolarization of the muscle cells and thus relaxation of the myocardium. During the depolarization phase, the myocardial cells are not responsive to stimulation, i.e., they are refractory. The electric potentials associated with depolarization and repolarization can be detected, and their characteristics recorded over time—known as an electrocardiogram—can be evaluated.
Such natural (intrinsic) events are detected by deriving the electric potentials of the myocardium of the respective cardiac chamber with the help of sensing electrodes, which are part of a corresponding electrode line. The sensing electrodes may at the same time be the stimulation electrodes, and may also be used as stimulation electrodes and as sensing electrodes in alternation. A pair of sensing electrodes including two neighboring electrodes, namely a tip electrode and a ring electrode, where the tip electrode also serves as a stimulation electrode, is typically used for sensing, i.e., detection of intrinsic events. Bipolar derivation of an intracardiac electrocardiogram (IEGM) can be achieved in this manner. Sensing and stimulation in the ventricle are performed with the help of a ventricular electrode line, and stimulation and sensing in the atrium (right atrium) are performed with an atrial electrode line, each connected separately to a respective cardiac stimulator. In addition, a left-ventricular electrode line may also be provided, typically protruding through the coronary sinus and a lateral vein branching off from the former into the vicinity of the left ventricle, where it may have a small-area stimulation electrode and/or sensing electrode.
The sensing electrodes are connected to corresponding sensing units during operation of the cardiac stimulator, these units being designed to evaluate a respective electrocardiogram recorded via a sensing electrode (and/or a sensing electrode pair) and in particular to detect intrinsic atrial and/or ventricular events, i.e., natural atrial or ventricular contractions. This is accomplished, e.g., by threshold value comparison, i.e., an intrinsic event is detected when the respective intracardiac electrocardiogram exceeds a suitably predefined threshold value.
The respective intrinsic atrial heart rate (atrial frequency) and/or ventricular heart rate (ventricular frequency) can be derived from the frequency at which atrial and ventricular events follow one another, and tachycardias can be detected, for example.
In addition to the properties of the pacemaker already described above, an implantable cardioverter/defibrillator may also deliver a stronger current pulse to the heart, such that this pulse should not only stimulate (depolarize) a small portion of the myocardium but should also depolarize the largest possible amount of the myocardium and thereby make it refractory, to thereby interrupt a cycling stimulation of the myocardium, which is typical of fibrillations. Such a pulse is known as a defibrillation shock. It is typically delivered via a large-area defibrillation electrode in comparison with the stimulation electrode or sensing electrode.
Such a defibrillation electrode is often implemented in the form of a shock coil on the exterior surface of the electrode line in the respective cardiac chamber. For example, a ventricular electrode line may have a ventricular shock coil, in addition to a tip electrode or a ring electrode for stimulation and sensing, and may also have a proximal shock coil that is situated in the superior vena cava after implantation.
As a rule, a defibrillation shock is delivered when the cardiac stimulator detects a fibrillation, i.e., an irregular high-frequency intrinsic cardiac activity, which is also known as ventricular fibrillation, an event resulting in incomplete contraction of the respective cardiac chamber. Such a fibrillation is classified as a tachycardiac arrhythmia, which includes tachycardias (e.g., ventricular flutter) in addition to fibrillations. In contrast with fibrillation, tachycardia is regularly followed by a complete contraction of the cardiac chamber affected, but at a higher rate than would be physiologically appropriate. Such tachycardias can often be treated by antitachycardiac stimulation and do not require a defibrillation shock. Fibrillations are usually treated with a defibrillation shock.
For detection of ventricular fibrillation, a detection unit is typically provided as a component of a control unit of a cardiac stimulator; and is connected to the right-ventricular sensing unit and is designed to perform primary ventricular fibrillation detection on the basis of the right-ventricular intracardiac electrocardiogram (IEGM) derived using bipolar leads. If a predefined detection condition (X-of-Y criterion) is met, the detection unit indicates a ventricular fibrillation.
In tachycardiac arrhythmias of the ventricle, a distinction is made between supraventricular tachycardias (SVT) and ventricular tachycardias (VT) in the narrower sense. The latter have their origin in the ventricle itself, whereas supraventricular tachycardias have their origin in the atrium. For the treatment initiated after detection of a tachycardia, the type of ventricular tachycardia (ventricular tachycardia in the narrower sense (VT) or supraventricular tachycardia (SVT)) is significant.
Treatment of tachycardias and fibrillations by means of intracardiac electrotherapy can be performed in various known ways. It is known to apply antitachycardiac stimulation (antitachycardia pacing, ATP) in the form of overdrive stimulation in which stimulation pulses are delivered at a stimulation rate, which is increased in comparison with the prevailing intrinsic (tachycardiac) heart rate, or in the form of delivery of cardioversion shocks or delivery of defibrillation shocks, whereby the former usually have a lower energy than the latter. Defibrillation shocks should make the entire myocardium of the affected cardiac chamber refractory at the same time, and therefore temporarily unresponsive to stimulation to thereby interrupt a cycling stimulation of the respective myocardium.
Treatment of ventricular tachycardias (VT) in the narrower sense by overdrive stimulation within the context of antitachycardiac pacing (ATP) should have the result that a reentry cycle of stimulation of the myocardium, which is typical of ventricular tachycardias (VT), is interrupted by a stimulation pulse which occurs before the natural (intrinsic) stimulation of the affected cardiac chamber. This requires reliable detection of intrinsic ventricular contractions prior to triggering of the ATP.
The present invention is based on the problem that oversensing of ventricular events may occur under some circumstances, i.e., the corresponding ventricular sensing unit detects more presumably ventricular events than actually occur. In many cases, such oversensing results in delivery of inadequate defibrillation shocks or at least results in the start of charging operations due to a perceived interference signal in the right-ventricular electrode. The service lifetime of an ICD is significantly reduced by such charging operations.